1. Field of the Invention
The present invention is in the field of electro-optical imaging, and more particularly relates to systems for converting electrical signals into chromatic radiation for light-gate array decoding in a Flat Panel Display (FPD).
2. Brief Description of the Prior Art
In the prior art, the color Cathode Ray Tube (CRT) has been universally utilized for the conversion of electrical signals into monochromatic or polychromatic images. Its versatility, however, is hampered by its inherent characteristics of geometric distortion, package depth, high voltages, lack of uniform resoultion, susceptibility to shock, gross weight, and the apparent impracticality of achieving large (greater than 35 inches diagonal) or small (less than 2 inches diagonal) image surfaces without projection or optical reduction, respectively.
Of recent interest is the co-called Flat Panel Display (FPD) as is noted in commercial literature (1). This type of display is available today in several varieties (2) known as Gas Plasma (GPD), Electrophorescent or Electroluminescent (ELD), Vacuum Fluorescent (VFD), and Liquid Crystal (LCD).
One prior art LCD, U.S. Pat. No. 4,090,219 (Ernstoff, et al), utilizes sequential color field techniques, variable liquid crystal reflectivity, and active electronics at each pixel site to achieve color imaging. Such displays generally suffer from low image resolution due to slow pixel response, narrow viewing angles, and video bandwidth degradation related to sequential color field operation. System performance attainment is further complicated by the mechanics of color filter switching, the use of field-effect transistors and capacitors at each pixel site, and the requirements for various video shift registers, electric latching, and sample-hold circuitry.
Displays utilizing gas plasma (such as neon and argon ions) are in widespread use, basically as monochrome or tone-on-tone devices. Voltages to activate these gases are high (90-185 volts) compared to those utilized in modern integrated circuitry (15 volts or less, typically). Image refresh times employed (around 200 milliseconds) are considered too slow for standard video. While these devices are relatively thin (3 inches) as compared to the standard CRT, they suffer as the CRT from undersireable weight and, as glass vacuum tubes, are shock-susceptible. Commercially offered ELD and VFD devices, as with FPDs just discussed, have not been shown to be viable alternatives to the color CRT; suffering generally from a lack of orthochromaticity, with slow video response, low bandwidth, and an inability to achieve broad gray-scale intensity shadings.
A method different from all of the foregoing is taught in U.S. Pat. No. 4,170,772 (Bly), wherein vertical strips of alternating red, green, and blue phosphors are arranged across a common transparent front-plane electrode and sandwiched between a plurality of horizontal back electrodes. Upon application of the proper voltage(s) between some horizontal electrode and the front-plane, the sandwiched phosphors are caused to glow and appear as a series of red-green-blue dots repeated the full length of the energized horizontal line. An electrobirefringent light-valve (light-gate) column array, utilizing a type of PLZT Ceramic material in a quadratic (Kerr Cell) format, is placed between the viewer and the horizontal phosphor dot emissions through the front-plane, such that the light-valve columns each address a phosphor dot. When the columnar light-valves are caused to vary transmissivity in response to video signals and while being properly sequenced, an image results.
Phosphor materials are generally not as responsive to steady state current changes as they are to electron beam excitation under vacuum conditions and short high voltage pulses. Further, degradation effects due to charge migration when phosphors are excited by pulsed or steady-state D.C. require alternation of applied voltage polarity periodically as an alleviation; leading to additional switching means. Electrode spacing with transverse (Quadratic) electro-birefringent materials also becomes problematical when interfacing with peripheral drive circuit connections for computer displays and the like. For instance, to provide for 10 volt switching of PLZT Ceramic light-valve arrays, requiring 15,000 V/inch (6,000 V/cm) between transverse electrodes, minute electrode spacing of about 0.00067 inch (0.00170 cm) is required. The electrodes themselves, when utilizing 15% of the spacing, would be only 0.0001 inch in width with a density of 1,500 per inch. Accordingly, apart from small screen scientific, military, or specialized industrial application, broad utilization of PLZT modulated phosphor emission devices as color video imagers has not materialized.
The instant invention contributes to the solution of many of the problems found in the prior art as hereinbefore stated. Utilization of light generators (such as the Laser or LED) to directly emit chromatic radiation totally responsive to the video input signal(s) circumvents the need for CRT electron beam means and the attendant large geometries and high voltages. LEDs, in particular, allow for low video drive voltages (2-10 volts) while providing faster response (10 nanoseconds or less) than other FPD methods discussed. Further, the invention does not possess the complexities presented by active emitters and/or electronics at each pixel site. Through the employment of linear birefringent materials such as Lithium Niobate (LiNb03) in the light-gate decoder, reasonably accessible electrode spacing of 0.008 inch is provided while good image resolution (0.20mm pixel pitch) is maintained. A thin decoder (0.003 cm) utilizing this material provides for optical switching with less than 10 volts.
As will be shown in the preferred embodiment of the invention, extensive circuitry for latching, sample and hold, high voltage drive, and FET-Capacitor pixel site control is not required. Configured in the "solid-state", the embodiment comprises a thin, rugged and practical Flat Panel Display with fast video response for either monochromatic or multicolor imaging. Additional contributions to the art, through the ability of the invention to radiate selectively at various output surface points, enable multi-channel switched transmissions as may be employed for signal multiplex/demultiplexing.